The present invention relates to web handling apparatus having a system and method for controlling the print-to-print or cut-to-print registration of a series of regularly spaced, repeated images, printed on a moving web, as the web moves through a plurality of spaced apart stations that perform web handling operations, such as printing, cutting and the like, with respect to the images on the moving web. More particularly, the present invention relates to a print-to-print and/or cut-to-print registration system and method that may be used with a variety of web handling apparatus including especially a multi-station, web fed rotogravure printing press.
Gravure printing has been used for years and is recognized as being particularly useful for printing high quality, multi-colored images, such as found on commercial packaging, advertisements and the like, at relatively high speeds. In such multi-colored printing, images are printed in one color on a moving web at a first printing station and then the web passes through a drier section before proceeding to a second printing station where a second color is printed on the images. This printing and drying sequence is repeated at the various printing stations comprising the web-fed rotogravure printing press until a final, full color image is achieved. Some presses include a web cutting station as the last station in the press.
High quality multi-colored printing on web-fed rotogravure printing presses requires the maintenance of close registration between the adjacent images on the web. Failure to obtain this registration may result in a fuzzy or blurred image.
Even though the stations, including the printing stations, in rotogravure printing presses are mechanically linked so that the nipped rolls at each station are driven at the same speed, mis-register, as a practical matter, is a continuing problem with web-fed rotogravure printing presses. The art has long sought a simple, relatively inexpensive solution to image mis-register problems in rotogravure printing presses.
The previous systems used to minimize image mis-register in web fed rotogravure printing presses sought to sense errors in the relative positions of the images and then take steps to correct the sensed position errors as quickly as practicable. In other words, prior print-to-print and/or cut-to-print registration systems did not attempt to avoid mis-register of the images, but rather only sought to correct any mis-register after it had occurred and had been sensed.
Typically the prior systems sensed mis-register by sensing the relative positions of a series of marks printed on a side marginal edge of the moving web. This sensing required the use of spaced apart sensors positioned along the moving web as it passed through the printing press. The sensed information is analyzed (generally by a computer), and the system then attempts to make corrections to the web speed or the tension on the web to seek to bring the images on the web back into registration.
The prior systems, presently in use by the art, tend to be relatively expensive, not only in terms of the equipment that is needed to sense the mis-register and control the web, but also due to the cost of the side marginal edge of the web, that is needed for printing the information to be sensed, since generally this edge must be discarded after printing. Further, when a different image is to be printed on a press, the sensors must be repositioned at the same time as the rolls in the press are changed. This adds to the press set up time. Moreover, the presently used systems, even those employing state of the art equipment, still do not avoid some image mis-register. A typical example of such mis-register, that heretofore had to be tolerated by the art, is 0.06 inch.
In 1973, one of the inventors herein, John R. Martin, postulated six rules for controlling image registration on a moving web. According to these rules, print-to-print or cut-to-print registration is maintainable: (1) if all web drive points are non-slip; (2) if the web is uniform within spans; (3) if the web in any span is not subject to any intermediate forces; (4) if "gains" with respect to the web are reasonable; (5) if all drive points have a constant relative speed; and (6) if the web path between nips is constant. The last rule, rule 6, can be satisfied where all the drive and idler rolls are permanently located if the tension of the web entering the first printing station is constant. While the theoretical basis for these Martin rules was and remains sound, manufacturing a web-fed rotogravure printing press that will satisfy the requirements of these rules has not heretofore been possible.